Module mod_cgi

This module is contained in the mod_cgi.c file, and is compiled in by default. It provides for execution of CGI scripts. Any file with mime type application/x-httpd-cgi will be processed by this module.

Summary

Any file that has the mime type application/x-httpd-cgi or handler cgi-script (Apache 1.1 or later) will be treated as a CGI script, and run by the server, with its output being returned to the client. Files acquire this type either by having a name ending in an extension defined by the AddType directive, or by being in a ScriptAlias directory.

When the server invokes a CGI script, it will add a variable called DOCUMENT_ROOT to the environment. This variable will contain the value of the DocumentRoot configuration variable.

CGI Environment variables

The server will set the CGI environment variables as described in the CGI specification, with the following provisions:

REMOTE_HOST: This will only be set if the server has not been compiled with MINIMAL_DNS.
REMOTE_IDENT: This will only be set if IdentityCheck is set to on.
REMOTE_USER: This will only be set if the CGI script is subject to authentication.

CGI Debugging

Debugging CGI scripts has traditionally been difficult, mainly because it has not been possible to study the output (standard output and error) for scripts which are failing to run properly. These directives, included in Apache 1.2 and later, provide more detailed logging of errors when they occur.

CGI Logfile Format

When configured, the CGI error log logs any CGI which does not execute properly. Each CGI script which fails to operate causes several lines of information to be logged. The first two lines are always of the format:

  %% [time] request-line
  %% HTTP-status CGI-script-filename

If the error is that CGI script cannot be run, the log file will contain an extra two lines:

  %%error
  error-message

Alternatively, if the error is the result of the script returning incorrect header information (often due to a bug in the script), the following information is logged:

  %request
  All HTTP request headers received
  POST or PUT entity (if any)
  %response
  All headers output by the CGI script
  %stdout
  CGI standard output
  %stderr
  CGI standard error

(The %stdout and %stderr parts may be missing if the script did not output anything on standard output or standard error).

Directives

ScriptLog

Syntax: ScriptLog filename
Default: none
Context: resource config
Status: mod_cgi

The ScriptLog directive sets the CGI script error logfile. If no ScriptLog is given, no error log is created. If given, any CGI errors are logged into the filename given as argument. If this is a relative file or path it is taken relative to the server root.

Note that script logging is meant to be a debugging feature when writing CGI scripts, and is not meant to be activated continuously on running servers. It is not optimized for speed or efficiency, and may have security problems if used in a manner other than that for which it was designed.

ScriptLogLength

Syntax: ScriptLogLength size
Default: 10385760
Context: resource config
Status: mod_cgi

ScriptLogLength can be used to limit the size of the CGI script logfile. Since the logfile logs a lot of information per CGI error (all request headers, all script output) it can grow to be a big file. To prevent problems due to unbounded growth, this directive can be used to set an maximum file-size for the CGI logfile. If the file exceeds this size, no more information will be written to it.

ScriptLogBuffer

Syntax: ScriptLogBuffer size
Default: 1024
Context: resource config
Status: mod_cgi

The size of any PUT or POST entity body that is logged to the file is limited, to prevent the log file growing too big too quickly if large bodies are being received. By default, up to 1024 bytes are logged, but this can be changed with this directive.